Since 1970 the global use of energy has increased by as much as 70% and the greenhouse gas emissions have increased by as much as 75%. It is thus necessary to reduce the emissions of CO2, SOX, NOX, particulate matter, and hydrocarbons, typically generated from coal and petroleum coke based processes, which lead to air, water, and soil pollution and cause drastic climate changes.
Gasification is a process which comprises reacting a carbonaceous material at high temperature with a controlled amount of steam and oxygen/air, to produce a gas mixture, called syngas or synthesis gas, containing predominantly carbon monoxide, hydrogen and carbon dioxide, which is used as fuel. The synthesis gas can be used for heat production and for generation of mechanical and electrical power. The synthesis gas can also be used for further processing to liquid fuels or chemicals. The high-temperature gasification process provides a more efficient and cleaner gas production making the process environmentally acceptable over the conventional combustion processes.
Although the gasification process is an old known technology, its commercial use has not been widely exploited through out the world because of the high costs involved due to extreme operating conditions and high endothermic heat demand. In the recent past, however, the gasification process has received good research attention because of the current crude market scenario.
With the increasing demand of petroleum and the development of deep resid processing technology through coking, the output of petroleum coke as a by-product from the petroleum refinery has significantly increased. It's a challenging task to utilize petroleum coke in a reasonable, efficient and clean way, since coke is a low value refinery product, however, due to its high calorific value and carbon content compared to coal, petroleum coke can be a preferred feedstock for the gasification process for producing synthesis gas.
However, the gasification activity of petroleum coke (high carbon content feedstock) is much lower than that of lignite/sub-bituminous coal (high reactivity carbonaceous feedstock), which greatly restricts its use as feedstock for the synthesis gas production. The gasification of petroleum coke is complicated due to its lower gasification kinetics, which demands higher temperatures than the high reactivity coal. In addition, high-sulfur and metal contents of the petroleum coke, are barriers for its specialty applications e.g. anode and needle coke. Petroleum coke (petcoke) is solid and its transportation from the refinery is expensive. It is therefore desirable to convert the low valued petroleum coke into a more usable energy source such as synthesis natural gas (SNG), synthesis gas or other high calorific value gases, which are freely transportable through the existing infrastructures such as pipe lines.
In order to obtain synthesis gas, most of the commercial gasifiers (such as entrained flow gasifiers) use pure oxygen. This demands additional capital and operational expenditures for air separation units. The process frequently encounters operational problems with reactor refractory/metallurgy and slag handling issues, etc., because of the severe operating conditions (T˜1400° C., P>30 bars). Other commercial gasifiers have been developed based on the fluidized bed technology in which the carbon conversion is relatively low compared to the entrained flow gasifiers because of their low operation temperature (i.e. fluidized gasifier operates in the temperature range between the ash softening and melting point temperatures). If the gasification temperature in the fluidized bed gasifiers is close to 1000° C., the ash content of the carbon feedstock starts to soften and the individual particles begin to agglomerate. The larger sticky particles fall to the bottom of the bed which reduces the gas permeability and tends to block the reactor and the reactor feed lines and their removal poses a considerable problem. Generally, both combustion and gasification reactions occur in the same vessel wherein part of coal/coke gets combusted at the bottom to supply endothermic heat for gasification that occurs at the upper part of the gasifier. Several operational issues in a single fluidized bed gasifier are experienced such as generation of hotspots, agglomeration, etc. If air is used as the combustion agent in the fluidized bed gasifier, the calorific value of the synthesis gas so produced will be low as N2 will dilute the synthesis gas.
The afore-said problems can be eliminated by carrying out the combustion and the gasification reactions in different fluidized bed vessels. The dual fluidized bed process is capable of producing synthesis gas with air instead of pure oxygen. In the dual fluidized bed process, two fluidized beds (namely, combustor and gasifier) are operatively connected to each other. Gasification of the carbon feedstock occurs in the gasifier and the endothermic heat required for this reaction is supplied by the separate combustion of unreacted carbon from the gasification chamber along with some make-up carbon in the combustor. The energy released during the combustion process is conveyed to the gasifier along with a circulating catalyst. The success of this process scheme depends upon the acceleration of the gasification kinetics of the feed stock in the presence of the catalyst, and the operating conditions such that the temperature difference between the two zones allows efficient heat transport by circulation of the catalyst. Catalytic gasification where catalyst is impregnated in the feedstock has the kinetic advantage of low temperature operation, avoiding many corrosive material formation and other operational constraints. A major drawback of the catalytic gasification process is catalyst regeneration, which is not yet completely resolved. Generally, the alkaline catalyst is recovered from the spent solids by water leaching where only a portion of the alkali can be recovered and an excess of make-up alkali is required which leads to increase in the operating costs.
In the known art there is no efficient catalytic dual fluidized bed gasification process which can operate at a temperature range where operational issues such as low carbon conversion, agglomerations, substantial catalyst loss from the bed, and catalyst regeneration, and the like, are avoided. In view of the above, there is scope to improve the existing catalytic gasification processes by performing combustion and gasification in the presence of a highly efficient catalyst at substantially lower temperatures in separate circulating fluidized bed vessels.
Several efforts have been made in the past to improve the gasification process, some of the known technologies and methods are listed below:
U.S. Pat. Nos. 4,157,245 and 4,391,612, US Application 2010/0181539, and publications by Sudiro et. al., and Christoph et. al., all disclose dual bed gasification processes.
U.S. Pat. No. 4,157,245 discloses a non-catalytic dual fluidized bed concept for countercurrent plug-flow of two solids i.e. a carbonaceous solid and a heat carrier (i.e. sand) which is circulated between the beds. The combustion and gasification is conducted in different vessels with countercurrent plug-flow of solids. The temperature difference between the combustor and the gasifier decides the circulation rate of the heat carrier. In order to maintain the low ratio of heat carrier to coke (<15) in the non-catalytic dual fluidized bed system, it is necessary to maintain the operating temperature of the combustor dose to the ash melting point temperature, which might cause severe problems in the fluidized bed such as coking, agglomeration, reduction of gas permeability, blockage of reactor internals, etc. On the other hand, at lower gasification temperatures, reactivity of high carbon content feed stocks such as petcoke, bituminous, and anthracite, etc. is very less and the presence of catalytic active sites are necessary to get substantial gasification at low temperatures. Hence, the non-catalytic dual fluidized bed process scheme is not suitable for either high carbon content or higher ash content carbonaceous feed stocks.
U.S. Pat. No. 4,391,612 discloses a dual bed concept for the catalytic gasification of carbonaceous solids, in which a fluidized bed reactor and an entrained flow lift riser are used for gasification and combustion, respectively. Extreme operating temperatures are proposed for combustion and gasification zones, i.e. 1250° C. (900 to 1300° C.) and 850° C. (700 to 1050° C.), respectively, which might lead to severe operating problems such as agglomeration and caking of the carbonaceous solids. The disclosure does not discuss the operational issues arising out of high temperature fluidized bed gasification. The catalyst (i.e. lime) is impregnated on coal, therefore, catalyst recovery and reuse is a major problem. Additional expenses are involved in the recovery and processing of the catalyst. Further, use of lime catalyst does not give a significant increase in the gasification kinetics.
US2010/0181539 discloses a system for dual fluidized bed gasification. It consists of a primary dual fluidized bed loop which produces low quality synthesis gas containing excess levels of methane, higher hydrocarbons and tar. The gas is conditioned in a gasifier of secondary dual fluidized bed loop to produce higher quality synthesis gas. The catalytic heat transfer material, i.e. nickel supported by α-alumina (suitable for reforming of hydrocarbon and CO2 and shift activity of CO), is circulated between the combustor and the gasifier in both the primary and the secondary dual fluidized bed loops. In the secondary dual fluidized bed loop, the combustor temperature is in the range of 899° C. to 927° C. and the conditioning temperature in the range of 829° C. to 857° C., whereas in the primary dual fluidized bed loop the gasifier can be operated in the temperature range of 593° C. to 704° C. The temperature difference in both the primary and the secondary dual fluidized beds loops is in the range of 16° C. to 149° C. If the temperature difference between the two vessels is less than 70° C., a very high heat carrier circulation rate (>100 times) is required, which is not feasible. Primarily, this scheme is conceived for biomass feed and feedstocks such as coal or petcoke, and preferably operated in fluidized bed combustors at a temperature less than 850° C. to avoid the problems of caking and agglomeration. Though it teaches the use of attrition resistant supports such as α-alumina, the proposed catalyst i.e. Ni is not suitable for substantial gasification of the feed stocks such as petcoke or coal. Also, α-alumina has very low surface area, pore volume and accessibility which does not provide adequate catalytic surface. Furthermore, multiple loops of dual fluidized bed make the configuration extremely complex. It appears that the above said disclosure is more appropriate to fine tune and achieve the molar ratio of synthesis gas to suit feedstock for the Fischers-Tropsch synthesis process.
Sudiro et. al. [Energy & Fuels, (2008), 22(6)] have developed the Aspen-Plus model for the non-catalytic gasification of coal in a dual fluidized bed reactor, in which combustion is carried out at 980° C. in one reactor and gasification is performed at temperatures as low as 700° C. in the another reactor. The heat requirement in the gasification chamber is satisfied by heat carried through thermal vectors from the combustion chamber. Though, the model results are encouraging, the proposed operating conditions may not be suitable for other carbonaceous feedstocks such as petroleum coke, anthracite, bituminous, etc., as the gasification reactivity is negligible at this gasification temperature i.e. 700° C. A catalytic action is necessary to initiate the gasification for these feedstocks at this low temperature. The Aspen-Plus model is further modified by Sudiro et. al. (Energy & Fuels (2010), 24), by taking into account kinetics and mass transfers for both gas phase and char particles. Though a new gasification temperature of 860° C. is proposed, the operating temperature of the combustion zone, i.e. 990° C., leads to severe operational problems such as caking, agglomeration, etc., in the combustor. In addition, it is proposed to maintain a high heat carrier circulation rate (>50), which leads to decrease in the throughput. In order to increase the throughput and minimize the inert solid circulation rate, higher values of ΔT are required which can be possible only by conducting the gasification at lower temperatures as there is an upper limit on the combustor temperature to avoid the agglomeration.
Christoph et. al. (19th European Biomass Conference and Exhibition (2011), Berlin, Germany) disclosed a biomass gasifier based on the concept of non-catalytic dual fluidized bed gasifiers. In order to improve the fuel flexibility and overall efficiency of the process, it is proposed to replace the conventional bubbling bed gasifier design with turbulent fluidized bed regime having counter current solid flow. Therefore, the gas-solid contact can be increased significantly which helps to achieve higher gasification rates as well as higher efficiencies. Further, the temperature of the gasifier is reduced to 650° C. by the implementation of sorption enhanced reforming process which uses in-situ carbon dioxide capture by the bed material i.e. CaO. This provides sufficient delta temperature between the combustor and the gasifier and demands low circulation rate of the bed material. However, the proposed process conditions and bed material are only suitable for biomass. At this temperature (<650° C.), the gasification reactivity of feedstocks such as petroleum coke, and high quality coals such as anthracite and bituminous is negligible.
EU Patent 0024792, U.S. Pat. No. 4,475,925, US Application 2007/0083072 and 2009/0165380 and publication by Kikuchi et. al., disclose the use of catalyst for improving gasification of carbonaceous feedstock.
EU0024792 discloses a process in which methane, tar and higher hydrocarbons lean synthesis gas is produced from feedstock such as coal/coke in a single fluidized bed gasifier. In this disclosure, the impregnated coal, in which 5 to 50% of feed is K2CO3 or Na2CO3 catalyst, is gasified in presence of steam and O2 at a temperature between 650 to 790° C. and pressure between 3 to 14 kg/cm2. The major drawback of this process is that the critical issue of catalytic gasification, i.e. catalyst recovery and regeneration, is not addressed. The proposed process is not economical as the catalyst is impregnated on the coal, which necessitates a costly process for recovery and reuse.
U.S. Pat. No. 4,475,925 discloses a catalyst and a heat carrier for the gasification of carbonaceous solids in a dual bed gasifier. A mixture of petcoke and KNO3 (either by physical mixing or impregnation) and sintered bauxite are suitable for the agglomeration free gasification up to 950° C. This disclosure has given more attention on the upper limit of the reaction temperature for a given catalyst-heat carrier mixture. As the catalyst is mixed with the coke, though it may not form any agglomeration with the heat carrier, the catalyst loss and regeneration are major hurdles which have not been addressed in this disclosure.
US2007/0083072 discloses the use of alkali catalyst (˜5 times greater than the ash content of the coke) for steam gasification of impregnated petcoke at a temperature between 650-760° C. and pressure about 34 bars. The conditions favor the production of SNG directly. The disclosure demonstrates a method for managing the endothermic heat of steam gasification with the exothermic heat of methanation. As the catalyst is impregnated on the carbon feedstock, the regeneration of the entire catalyst is not possible. This therefore requires costly recovery of catalyst for reuse.
US2009/0165380 discloses a process for petroleum coke catalytic gasification at 700° C. and 34 atm pressure in a fluidized bed gasifier, which uses a catalyst (mixture of KOH and K2CO3) loaded on the coke for improving the gasification. This disclosure suggests a catalyst composition and operating conditions for the production of methane directly from the carbon feedstock. As the catalyst is impregnated on the coke, it escapes from the bed along with the product gas. The disclosure does not disclose the recovery and regeneration of the catalyst.
Kikuchi et. al. (ACS Fuel Volumes, (1984), 29 (2), 179-185) discloses the use of impregnated K2CO3 on alumina (having the structure of α-Al2O3) for the gasification of active carbon in a single fluidized bed gasifier. The kinetics of activated carbon in the presence of the catalyst and the effect of the catalyst loadings on the gasification rate are disclosed. The presented results are at the temperature of 850° C. with a catalyst composition of 17 wt % of K2CO3 on α-Al2O3. It is known that the surface area and pore volume of α-Al2O3 is less and sufficient catalyst dispersion cannot be obtained with α-Al2O3. It is concluded in the disclosure that the carbon conversion is independent of the catalyst to the coke ratio. It therefore appears that the gasification yields are mainly due to the higher gasification temperature (850° C.). It is known that the kinetics at high temperature are different than that at low temperature. The catalytic action on the gasification yield is significant at lower temperatures than at higher temperatures. Therefore, the catalyst used in the above study may not be suitable for achieving substantial catalytic gasification at lower temperatures (i.e. <750° C.). It is therefore highly desirable to bring down the reaction temperature to 750° C. with the help of suitable catalyst composition with proper support and loading such that the viability of the process increases tremendously.
US patent 2012/0046510 discloses a process for the hydromethanation of a carbonaceous feedstock in which superheated steam, hydromethanation catalyst, oxygen rich gas stream and carbonaceous material are fed to a single fluidized bed vessel that operates at high pressure (i.e., 30-60 bar), along with recycled synthesis gas stream. In order to meet the endothermic heat demand, it is proposed to combine the methanation reaction with the steam gasification and the overall reaction is expected to be thermally balance. However, due to the process heat losses and other energy requirements (such as evaporation of moisture in the feed stock) a small amount oxygen rich gas stream is proposed to be injected to the reactor for maintaining the thermal balance. Though it teaches efficient ways of achieving heat balance, as the catalyst (preferably alkali) is impregnated on carbonaceous feedstock, the catalyst recovery and regeneration demand additional complex process configurations which are capital intensive process.
In view of the above, although the use of dual bed gasifiers is reported in literature, most of them are for non-catalytic gasification of coal. The reported temperature between the two vessels, as mentioned for the gasification of coal, may not work for less reactive carbon feed stocks (i.e. conversion is very less at temperatures below 800° C.). In few prior arts, catalytic gasification of coal/coke by using dual bed gasifiers is reported. In these cases, catalyst is impregnated on the coal/coke or physically mixed with the carbonaceous solid for the steam gasification in dual bed fluidized gasifiers. The catalyst escapes from the fluidized bed rector along with the fly ash, as coal gets reacted. The fly ash therefore contains significant amounts of unconverted carbon and catalyst. Thus, the catalyst impregnated coke requires elaborate steps of catalyst recovery and reuse. Catalyst recovery and regeneration is always a major problem and often requires additional processes which lead to extra expenditures.
The supported catalyst as a separate solid particle in the fluidized bed gasifier is also reported in literature, however, a suitable catalyst or a proper support to obtain significant gasification at lower temperatures is not provided. A suitable gasification catalyst is therefore required for significant gasification at low temperatures and a proper support is required to obtain better dispersion of the active sites along with high attrition resistance. It is highly desirable to have a process scheme for the low temperature gasification of a variety of carbonaceous feed stocks in the presence of an appropriate catalyst that provides for making catalytic activity towards gasification, water gas shift reaction and methanation, etc., and adopt the dual bed gasification at substantially low temperatures in the gasification step. It is also expected to minimize or eliminate the issues of catalyst loss from the bed, as this catalyst acts as a separate particle and it remains within the bed while achieving near complete gasification of the carbon.